1. Field of the Invention
This invention relates to improved barrier layer coatings of oxides of silicon on the surface of articles molded from certain thermoplastic organic polymers.
2. Description of Related Art
U.S. Pat. No. 3,442,686 discloses multilayer structures as flexible transparent packaging film having good barrier properties to gases and liquids, comprising a flexible transparent organic polymeric base film having thereon an adherent, transparent, flexible highly gas- and liquid-impermeable, moisture resistant, continuous glassy coating of inorganic material, and a sealable, flexible, transparent top coating of organic polymeric material. The glassy coating is sandwiched between the base film and top coating. The preferred base films disclosed include polyester films such as Mylar.RTM. oriented, heat set polyester film. The preferred glassy coatings disclosed are the oxides of silicon and aluminum, and the coating thickness is disclosed to be 0.02 to 2 microns (20 to 2000 nm). Among the silicon oxides disclosed are silicon monoxide (SiO) and silicon dioxide (SiO.sub.2). The glassy coating is formed on the base film by vacuum deposition. Examples of sealable top coatings disclosed are polyethylene and vinylidene chloride/acrylonitrile copolymer.
SiO is known to have better barrier properties than SiO.sub.2, but unfortunately SiO is much more expensive than SiO.sub.2 and imparts a yellow color to the package in the coating thickness required for development of its good barrier properties.
Nevertheless, after all the years of passage after publication of U.S. Pat. No. 3,442,686 in 1969, a commercial product has finally been introduced (in Japan) in which the glassy coating is a layer of SiO sandwiched between two layers of polyethylene terephthalate (PET) film and a cast polyolefin top coating to form a packaging film. For one of the products, the SiO layer thickness is on the order of 2000 Angstroms (200 nm). Another product consists of a laminate in which two superimposed layers of SiO are present, each having a thickness of 1200 Angstroms (120 nm), giving a total effective SiO thickness of 2400 Angstroms (240 nm). These products have two disadvantages. First the product has a distinctly yellow color arising from the SiO layer(s) at the layer thicknesses involved, ie., SiO has a yellow color, which becomes more intense as its thickness increases. It is believed that the yellow color of the SiO coating is somewhat ameliorated by carrying out the vapor deposition of the coating in the presence of oxygen, whereby the SiO is on the order of SiO.sub.1.5 . Second, the packaging film constituting this commercial product has the disadvantage of high cost arising from the high thickness and high relative cost of SiO present in the packaging film to provide the needed barrier properties. To their credit, however, these packaging films are reported to be able to survive retorting at 125.degree. C. without loss of barrier property.
Effort has been made to use SiO.sub.2 coating instead of SiO for economy reasons and in order to obtain a colorless package. To some extent, the barrier properties of SiO.sub.2 can be improved by increased thickness, but there is a limit to this approach because of decreasing flexibility with increasing thickness.
U.S. Pat. No. 4,702,963 discloses a multilayer structure in the form of a packaging film in which an adhesion layer is first vacuum deposited on a flexible polymer substrate, followed by vacuum deposition of a barrier layer, to confer retortability to the packaging film. The adhesion layer can consist of Cr, which is preferred, Ta, Ni, Mo, oxides of Cr, alloys of Cr with Ta and Ni, co-deposited mixtures of Cr and SiO having at least 20% by weight of Cr and a lead-alumina-silica glass composition, and the adhesion layer is disclosed to have a thickness of 5 to 50 Angstroms (0.5 to 5 nm). The patent discloses that the barrier is preferably silicon monoxide or silicon dioxide, and that when silicon dioxide is used, it may be mixed with glass modifiers such as oxides of Mg, Ba, and Ca, or with fluoride of alkaline earth metals, e.g., MgF.sub.2 to alter the color appearance of the overall structure. A chromium/SiO composite film is disclosed to produce a coating with a yellowish appearance, while a neutral gray appearance is disclosed to result from the mixture of SiO.sub.2 with glass modifiers. The specific adhesion/barrier layer systems disclosed in the Examples are primarily chromium as the adhesive layer and SiO or SiO.sub.2 as the barrier layer. The lead-alumina silica glass is used as an adhesive layer, as is Ta-Cr alloy, Ta, Mo, and chromium oxides. In some Examples, the SiO.sub.2 layer is mixed with a modifier. In Table 4, small changes in adhesion layer thickness are disclosed to greatly diminish light transmission. In addition, retortability is determined from a simulated test in which silicon oxide layer adhesion is reported after retorting but not any barrier property of the multilayer structure. The approach taken in this patent has not achieved commercialization.
U.S. Pat. No. 4,528,234 discloses a transparent multilayer structure comprising a plastic film having a vacuum deposited layer thereon of Al, Sn, Fe, Zn and/or Mg and then a top layer of a carboxyl-containing polyolefin layer. The metal layer is thin enough that the structure is transparent and the metal layer effects a secure bond between the plastic film and the carboxyl-containing layer. The patent also discloses that a vacuum deposited layer of silicon oxide or titanium oxide on one or both sides of the plastic film improves the barrier properties of the structure and a package made therefrom is particularly suitable for retorting. In the Examples, both silicon monoxide and silicon dioxide are used as the barrier layer. These multilayer constructions have not achieved commercialization.
Japanese patent publication 62-156943 discloses a multilayer barrier structure of multiple layers of film and multiple layers of a vacuum deposited metal such as Al, Zn, Cu, Pt, In, Sn, Au, Ag, Sn or metal compound such as silicon oxide. The layers of vacuum deposited metal or metal compound are separated from one another by a layer of film and/or a layer of adhesive.
Thus there still remains the need to improve on the presently available commercial product of a coating of SiO on PET base film and top coated with cast polyolefin, insofar as providing equivalent barrier properties at lower cost and without the distinct yellow coloration arising from the SiO layer.